Change as much as you want, even until your fingers hurt. Note that there are no quests to get the new skills, unless they are class specific, meaning you have to buy them all. You can change in that PvP city, Bathazars whatever.
Btw, you have to be ascended (in prophecies), or closer to the stars (in factions), before you can swap.
Prophecies & Factions also have different ways to gain the ability to change professions. In Prophecies, each profession has a quest you need to do before you can change to that profession the first time. In Factions, it cost you 500g to change to a given profession the first time. (i.e.: start with a W/Mo - change to a W/R = 500g. Change again to W/Me = 500g, Change back to W/M or W/R = no charge!)